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Advertising division: IEK-4 - Plasma Physics
Reference number: 2019M-008, Physics

Master Thesis: Wf/W yttrium oxide interface production by chemical sol-gel process and interface fracture resistance study


In future fusion reactors, tungsten is a main candidate material for the first wall material of the divertor. The intrinsic brittleness of tungsten is, however, a concern with respect to the fusion environment with high transient heat loads and neutron irradiation. To overcome this drawback, a tungsten fiber-reinforced tungsten (Wf/W) composites are being developed relying on an extrinsic toughing mechanism. For Wf/W composites, a weak interface layer (normally a ceramic material) between the tungsten fibers and tungsten matrix plays a crucial role for the required pseudo ductile behavior.

Two potential production path of the Wf/W are: Chemical Vaper Infiltration (CVI) and Powder Metallurgy (PM). For both cases, the interface is currently produced by a magnetron sputtering process. However, with high time and material consumption, the production rate of the interface coating is relatively low. This makes the interface coating production becomes the bottle neck of the Wf/W composite manufacturing route. To solve this problem, we need to develop a new coating process which has both high production rate and superior coating quality. The Sol-gel process, as a commercially mature coating process, is one potential solution [1-3]. From experience with preliminary tests of the sol-gel process, it seems possible to achieve the coating material. However, the coating quality needs to be optimized.

Additionally, the interfaces mechanical characterization is currently another important topic for our Wf/W material development to have a better understanding of the reinforcement mechanism. One potential way to measure the fracture resistance is a so called “notched four point bending test with symmetrical interfacial cracks” [4-6]. With this method, the mechanical properties of the interfaces, that are produced by different coating processes (magnetron sputtering and sol-gel process) and different production routes (CVI and PM), can be characterized.

In this master thesis, the sol-gel process shall be optimized to produce the high quality interface coating (mainly yttria) needed. Especially the process parameters influence on the coating quality and coating structure will be studied. The coating can be firstly prepared on flat surface and then eventually on the long and short tungsten fibers. Here next to the coating production the characterization methods need to be implemented, including XRD, LM, SEM (FIB, EDS).

In addition to the sol-gel process optimization, the “notched four point bending test with symmetrical interfacial cracks” need to be prepared and evaluated. The influence of the coating technics and production routes on interface mechanical properties can then be studied. The specimens shall be prepared by using different coating technics (magnetron sputtering and chemical sol-gel process) and production routes (CVI, SPS and HIP).

Work plan

1 monthLiterature study about Wf/W interface material, sol-gel process and interface characterization
Prepare for the production and characterization processes
2-8 monthProduction of the interface on flat surface by sol-gel process with different parameters on flat surface.
Coating characterization.
Interface production on fibers.
Sample preparation for interface resistance measurement
Interface resistance measurement and calibration
9 monthSummary and thesis writing.
10 monthPresentation and exam.

[1] R.P. Rao, Growth and characterization of Y2O3:Eu3+ phosphor films by sol-gel process, Solid State Commun., 99 (1996) 439-443.
[2] M.K. Chong, K. Pita, C.H. Kam, Thermal annealing effect on Y2O3:Eu3+ phosphor films prepared by yttrium 2-methoxyethoxide sol–gel precursor, Materials Chemistry and Physics, 100 (2006) 329-332.
[3] C.-Y. Tsay, C.-H. Cheng, Y.-W. Wang, Properties of transparent yttrium oxide dielectric films prepared by sol–gel process, Ceramics International, 38 (2012) 1677-1682.
[4] P. Charalambides, J. Lund, A. Evans, R. McMeeking, A test specimen for determining the fracture resistance of bimaterial interfaces, Journal of applied mechanics, 56 (1989) 77-82.
[5] P.G. Charalambides, H.C. Cao, J. Lund, A.G. Evans, Development of a test method for measuring the mixed mode fracture resistance of bimaterial interfaces, Mechanics of Materials, 8 (1990) 269-283.
[6] A.G. Evans, M. Rühle, B.J. Dalgleish, P.G. Charalambides, The fracture energy of bimaterial interfaces, Materials Science and Engineering: A, 126 (1990) 53-64.

Dr. J.W.Coenen
Forschungszentrum Jülich
Institute of Energy and Climate Research - Plasma Physics (IEK-4)

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